Subseasonal Coastal‐Trapped Wave Propagations in the Southeastern Pacific and Atlantic Oceans: 1. A New Approach to Estimate Wave Amplitude

Abstract

AbstractThe Humboldt and the Benguela upwelling systems are connected to the equatorial variability through the coastal waveguide, so that a large variance of the coastal sea level and current variability can be described as an infinite sum of orthonormal free Coastal‐Trapped Wave (CTW) modes. The objective of this study is to infer the CTW mode contributions to the coastal variability in both systems at subseasonal timescales (<120 days) from regional ocean circulation model simulations. We develop and validate twin regional model configurations of the southeastern Pacific and Atlantic Oceans. Cross‐shore spatial structures of the first four free CTW modes are then derived from model mean stratification and topography along the southwestern African and South American continents. We introduce and validate a new methodology to estimate the gravest CTW mode contributions to model pressure and alongshore current. Our formulation draws on the orthonormality of the CTW modal structures, and uses a simple projection of the coastal and bottom model pressure onto each CTW structure. Results give confidence in the ability of this modal decomposition methodology to disentangle CTW mode contributions from complex nonlinear coastal processes that control the coastal subseasonal variability. In both systems, it allows to successfully extract the gravest poleward propagating CTW modes with velocities close to the theoretical values and amplitudes consistent with the solutions of a simple multimode linear CTW model. Furthermore, results show that both systems exhibit relatively different CTW dynamics and forcings which are discussed in the companion paper (Illig et al., 2018).